1. Field of the Invention
The present invention relates to hard tissue repairing materials and, particularly, to a bone repairing material that is used to repair when articular function and/or bone function of hands and feet are lost. Furthermore, the invention relates to an implant that can be used as an artificial tooth, for rebuilding the teeth and tusks when the teeth had been lost because of senility or illness.
2. Description of the Prior Art
In the case of damage in a hard tissue repairing material, e.g., a bone and a tooth, any artificial hard tissue repairing materials may be inserted in living body for treatment. The hard tissue repairing material has to be bonded to the living hard tissue in living body after having been inserted. In order to bond the hard tissue repairing material to a living hard tissue, the hard tissue repairing material should have any functional group capable of inducing the nucleation of hydroxyapatite so that hard tissue repairing material should have a xe2x80x9cbone-like apatitexe2x80x9d on its surface. The bone-like apatite is the hydroxyapatite which has carbonate ion (CO32xe2x88x92) and low Ca ion concentration (Ca deficiting) regarding with stoichiometric composition (Ca10(PO4)6(OH)2). The bone-like apatite also has a Ca/P ratio that is lower than 1.67 of the stoichiometric hydroxyapatite. The bone-like apatite has a plurality of lattice defect and is constructed by fine particles. Therefore, the bone-like apatite is nearly equal to the bone apatite of living bone.
It is known that a hard tissue repairing material including a base material, e.g., metal or ceramics, and a zirconia gel layer made by a sol-gel process. The zirconia gel layer has a Zrxe2x80x94OH group that may induce a nucleation of apatite. (See, Bioceramics volume 11 Ed. by R. Z. LeGros and J. P. LeGros, World Scientific, (1998) pp77-80).
By the way, several prior art methods of giving the bioactive function at the surface of a base material are disclosed in some literature. For example, the Japanese Laid-open Patent Publication No. 6-23030 discloses a method of forming a coating layer of silica gel or titania gel on the surface of a base material. The Japanese Laid-open Patent Publication No. 10-179718 discloses a method of improving the surface of a base material of titanium metal and titanium alloys to bioactive by soaking in an alkaline fluid.
The layer having the hydroxyl group formed by the method of the Japanese Laid-open Patent Publication No. 6-23030 is a silica gel layer or a titania gel layer on the surface of the base materials. Similarly, the bioactive layer formed by the method of the Japanese Laid-open Patent Publication No. 10-179718 is titania phase, titania gel phase, alkaline-titanate phase, and alkaline-titanate gel phase
However, the prior art hard tissue repairing material has a low level ability of inducing the nucleation of hydroxyapatite and, accordingly, the prior art hard tissue repairing material can not be bonded greatly to a living hard tissue in living body.
Therefore, it is an object of the present invention to provide a hard tissue repairing material having a high bioactivity.
In accordance with one aspect of the present invention, there is provided a hard tissue repairing material including a base material and a surface layer having a crystalline zirconia covering the base material, which the surface layer includes a Zrxe2x80x94OH group.
The hard tissue repairing material includes a surface layer having a Zrxe2x80x94OH group to induce a nucleation of apatite. Additionally, the hard tissue repairing material of this invention can form large amount of apatite, so that the hard tissue repairing material may have a good bioactivity. We consider that the hard tissue repairing material includes a surface layer having the crystalline zirconia so that the apatite can grow as maintaining the coordination between the crystalline direction of the Zrxe2x80x94OH group and the crystalline direction of the OH group of apatite. The surface layer having the crystalline zirconia may have a thickness ranging from 0.5 to 50 xcexcm, preferably thickness ranging from 1 to 10 xcexcm, more preferably thickness ranging from 1 to 5 xcexcm.
Preferably, a middle layer between the base material and the surface layer, which includes at least one element of the base material and at least one element of the surface layer having a crystalline zirconia, may be formed. With the middle layer, the contact between the base material and the surface layer having the crystalline zirconia can have a good contact strength. The middle layer may include an amorphous phase or crystal phase. Additionally, the middle layer may include a composite, e.g., double salt, or solid solution including at least two elements.
The surface layer may have a tetragonal zirconia, a monoclinic zirconia, or the both. Preferably, the surface layer may contain at least an ionic component that is selected from the group consisting of calcium ion, sodium ion, potassium ion, and phosphate ions. Consequently, the ionic component within the surface layer can promote a bone-like apatite. More preferably, the hard tissue repairing material may further include a second layer containing an apatite layer as a main component formed on the surface layer having the crystalline zirconia. The apatite layer in the second layer can promote the apatite spontaneously in the living body.
The base material may be a ceramic material including an oxide group, a carbide group, a nitride group, or a boride group. Additionally, e.g., silica-glass, preferably zirconia, zirconia-alumina composite may be used. The base material may be a metal material, e.g., titanium, Coxe2x80x94Crxe2x80x94Mo alloy, and may be a polymer material.
A process for producing a hard tissue repairing material includes preparing a base material, coating the base material with a zirconia sol, and crystallizing the zirconia sol.
The zirconia sol solution may include zirconium alkoxide, alcohol, distilled water, acid catalyst. Preferably, a solution including Zr(OC3H7)4, C2H5OH, H2O and HNO3 may be used as zirconium sol solution. Soaking the base material in the zirconia sol solution may result in coating of the base material with the zirconia sol. Additionally, crystallizing the zirconia sol may be effected by heating. It is noted that coating the zirconia sol and heating the base material having the zirconia sol on the surface may be repeated. According to desired repeats of coating and heating, the surface layer having the crystalline zirconia may have a thickness ranging from 0.5 xcexcm to 50 xcexcm, preferably from 1 xcexcm to 10 xcexcm, and more preferably from 1 xcexcm to 5 xcexcm.
The heating temperature is not limited, provided that the crystalline zirconia can be formed by heating. The temperature will be dependent on any conditions, e.g., the composition of the zirconia sol solution and/or the atmosphere. In the case that the zirconia sol solution including Zr(OC3H7)4, C2H5OH, H2O and HNO3 is used, the base material having the zirconia sol on the surface may be heated at not lower than 500xc2x0 C. in air. More preferably, the base material may be heated at not higher than 800xc2x0 C. If the base material is heated under other condition, e.g., in a hydrothermal bath, the base material may be heated at much lower than 500xc2x0 C.
A process for producing a hard tissue repairing material includes preparing a base material, coating the base material with a zirconia sol, heating the base material having the zirconia sol on the surface. Then, the diffusing layer having at least an element of the base material and at least an element of the zirconia sol is formed on the base material. Subsequently, coating the diffusing layer with a zirconia sol, and crystallizing the zirconia sol on the diffusing layer. Therefore, the surface layer having a crystalline zirconia is formed, and a middle layer between the base material and the surface layer is formed by the diffusion layer.
The heating condition to form the middle layer is not limited, provided that the element can diffuse. The heating condition may be changeable according to the base material. For example, where the ceramic material, e.g., silica-glass, zirconia, zirconia-alumina composite is used as base material, the base material having the zirconia gel on the surface may be heated at not lower than 1000xc2x0 C. If the metal, e.g., titanium, is used as base material, the base material may be heated within an inert gas, e.g., N2, Ar gas in order to prevent the oxidation. Additionally, the base material may be heated at not higher than 800xc2x0 C. in order to prevent the phase transition from alpha phase to beta phase.
The base material may include a hydrophilic group, e.g., a hydroxyl group. According to the hydrophilic group of the base material, the surface layer can have a good contact strength to the base material, because the hydrophilic group of the base material may be bonded to the hydroxyl group of the surface layer by dehydration and condensation. The base material may be soaked in an alkaline aqueous solution or an acid aqueous solution after the step of preparing the base material to provide a hydrophilic group on the surface.
Preferably, the zirconia sol may include at least an ionic component selected from the group consisting of calcium ion, sodium ion, potassium ion, and phosphate ions so that the surface layer may include the ionic component. In order to include the ionic component in the zirconia sol, a compound having the desired ion may be added in the zirconia sol solution. The compound may be a metallic hydroxide, e.g., calcium hydroxide, sodium hydroxide, potassium hydroxide, and an alkoxide, e.g., calcium ethoxide, sodium ethoxide, potassium ethoxide, and a nitrate, e.g., sodium nitrate, calcium nitrate, potassium nitrate, and acetate, carbonate, chloride, phosphate.
After crystallizing the zirconia sol, the process for producing a hard tissue repairing material may further include the step of soaking the base material in a molten salt containing at least an ionic component selected from the group consisting of calcium ion, sodium ion, potassium ion, and phosphate ions.
After crystallizing the zirconia sol, the process for producing a hard tissue repairing material may further include the step of soaking the base material in an aqueous solution containing at least an ionic component selected from the group consisting of calcium ion, sodium ion, potassium ion, and phosphate ions.
A process for producing a hard tissue repairing material may further include the last step of soaking the base material including the surface layer having the crystalline zirconia in the simulated body fluid with ion concentrations nearly equal to those of human body fluid. The simulated body fluid (called xe2x80x9cSBFxe2x80x9d hereunder) may be, e.g., the solution including Na+ 142 mM, K+ 5.0 mM, Mg2+ 1.5 mM, Ca2+ 2.5 mM, Clxe2x88x92 147.8 mM, HCO3xe2x88x92 4.2 mM, HPO42xe2x88x92 1.0 mM, SO42xe2x88x92 0.5 mM.
According to the hard tissue repairing material of this invention, it has a surface layer having the crystalline zirconia to form apatite layer. Therefore, the hard tissue repairing material can have a good bioactivity. Additionally, according to the process for producing the hard tissue repairing material of this invention, the hard tissue repairing material having a good bioactivity can be provided.